This invention relates to a liquid level sender assembly for a float operated liquid level sensor. In one aspect, it relates to a liquid level sender assembly for monitoring the level of fuel in the fuel tank of a motor vehicle.
It is well known to measure the level of liquid in a tank using a sensor having a pivotable float which controls the movement of a wiper contact over a resistive track. Electrical connections to the wiper contact and the resistive track produce an electrical circuit having a variable resistance which corresponds to the position of the float. As the float moves in accordance with the level of liquid in the tank, the resistance of the circuit can be electrically sensed to provide an indication of the liquid level. The portion of the sensor incorporating the resistance track and wiper contact is commonly known as a sending unit or sender assembly. The sender assembly typically does not include a float or float arm but is adapted for the subsequent connection of such items.
Liquid level sender assemblies of the type described above are widely used to measure fuel levels in motor vehicles including cars, trucks, buses, and boats. However, despite their widespread use, prior art sender assemblies are subject to several disadvantages. Two of the most significant disadvantages are float-arm-induced contact distortion and contact dither, as further explained below.
Float-arm-induced contact distortion occurs when side forces on the float arm caused by sloshing fuel, vehicle acceleration, gravity and the like are transmitted from the float arm to the wiper contact, causing the wiper contact to distort and change the contact force exerted on the resistive track. It is known that changes in the contact force, i.e., the normal force exerted by the wiper contact pressing against the resistive track, can produce unwanted changes in the resistance output of the sender assembly circuit. Therefore, in order to provide an accurate liquid level reading, it is desirable for a sender assembly to maintain constant contact force as the float arm moves, despite external loads on the float arm.
Contact dither (i.e., trembling) occurs when the wiper contact oscillates for a prolonged period at low amplitude about a single point on the resistive track. Dither is frequently caused by vibrating fuel which causes the float arm to oscillate at a single fuel level (e.g., during transportation of the vehicle on a trailer). Contact dither can cause abnormal wear of contact surfaces of the wiper contact and/or resistive track at the point of the dithering, which in turn can change the resistance of the sender assembly circuit and result in erroneous level indications. In order to provide accurate liquid level readings and prolonged sender assembly life, it is desirable for a sender assembly to reduce or eliminate contact dither.
Prior art sender assemblies are known which rigidly constrain the movement of the contact assembly to minimize float-arm-induced contact distortion, however, these sender assemblies are subject to contact dither. Other prior art sender assemblies are known which utilize an external contact carrier to reduce contact dither, however, these sender assemblies are subject to float-arm-induced contact distortion. A need therefore exists, for a sender assembly which minimizes both float-arm-induced contact distortion and contact dither.
Prior art sender assemblies are also subject to pivot distortion which can interfere with the proper operation of the float arm. Pivot distortion is often caused when the attachment end of a metallic (e.g., wire) float arm is inserted directly into the pivot hub of a sender assembly. If the inserted portion of the float arm has any residual curvature (e.g., from the manufacturing process) which is forcibly straightened during insertion into the pivot hub, this results in a constant bending force applied to the inside of the hub. This bending force can cause the pivot hub to distort over time (especially if the hub is made from a plastic material), which can lead to binding of the pivot hub and abnormal operation. Alternately, the attachment ends of the pivot arms must be carefully manufactured to avoid any residual curvature, which can cause additional expense. A need therefore exists, for a sender assembly which is not subject to pivot distortion even if metallic float arms are used which have some residual curvature in the attachment end.
Since the ultimate application of a liquid level sender assembly is not always known prior to manufacture, it is desirable to provide a sender assembly adapted to allow connection of a float arm/float assembly at a later time to form a complete sensor. Also, it is desirable that the connection resist twisting moments caused by forces on the float. Further, while the connection should resist accidental detachment of the float arm, it is desirable that the float arm be easily removable (on purpose) if requirements change. Still further, it is desirable that the connection require a minimum of parts, labor and equipment for attachment and detachment.
Prior art sender assemblies are known which have a longitudinal hole in the pivot hub for inserting a portion of the float arm sized to produce a tight interference fit therein. Such assemblies resist twisting and accidental detachment but require equipment such as jigs and presses for attachment of the float arm to the pivot and do not allow the easy removal of the float arm if requirements change. Other prior art sender assemblies are known which have a longitudinal hole in the pivot hub for accepting a float arm and a snap-on attachment member on a pivot arm for securing the float arm. Such assemblies resist twisting and accidental detachment, and allow removal if requirements change, however, the insertion of the float arm into the pivot hub can lead to pivot distortion as previously discussed. Still other prior art sender assemblies utilize a connection having a shaft held by a set screw for attaching the float arm. Such assemblies allow easy removal of the float arm, but can allow twisting or accidental detachment if the set screw is not adequately tightened or vibrates loose. Further, use of a set screw involves extra parts, labor and tools to manufacture and install. A need therefore exists, for a sender assembly which is adapted to allow post-manufacture connection of a float arm/float assembly, which resists twisting moments caused by forces on the float, resists accidental detachment of the float arm, allows easy removal of the float arm, and requires a minimum of parts, labor and equipment for attachment and detachment.
In one aspect of the present invention, a liquid level sender is provided which has a housing that has a resistive track mounted thereto. Pivotably connected to the housing about a pivot axis are a pivot arm and a contact carrier. The pivot arm has a rotational force transmitting member and the contact carrier has a rotational force receiving member. The force transmitting member cooperates with the force receiving member such that only force components directed circumferentially about the pivot axis are transmitted to the contact carrier. A contact assembly is connected to the contact carrier, and is electrically cooperative with the resistive track to provide a variable resistor.
In a preferred embodiment, the force receiving member is a boss and the force transmitting member is two spaced apart surfaces on either side of the boss. Preferably, the distance between the spaced part surfaces is greater than the boss to provide play to avoid contact dither.